1. Field of the Invention
The present invention is in the field of emission mioroscopy, and more specifically, integrated circuit inspection systems.
It is well known that in integrated circuit (IC) operation, current conduction through a damaged dielectric can cause it to emit extremely faint light. These photo emissions can be detected by the emission microscope disclosed in U.S. Pat. No. 4,680,635 to Khurana, as follows: The IC or "Device Under Test" (DUT) is placed on the microscope stage with the DUT area to be inspected centered in the axis of the optic system and camera. A light-tight chamber is closed around the microscope, the DUT is illuminated, and, while being viewed through the CRT display by the operator, positioned with the area of interest in the microscope axis. The Z axis elevation of the stage is manually adjusted for better focus if necessary. First, without applying power, the (bright or dark field) illuminated DUT is imaged through the video camera to obtain a "reflected" light top view image of the structural pattern of the DUT. The reflected image is converted into digital form and stored (in memory). Second, the illumination is turned off, and without applying power, any (thermal emissions) background noise light from the inspection area is collected (possibly integrated) and amplified in the analog video camera, and optionally in the digital image computer, to obtain a "background" image, which is digitized and stored. Third, a failure condition "test vector" of voltages (under which a defect in the DUT has previously been detected by an automatic test equipment (ATE) system) is applied by manual switches to the I/O terminals of the (still unilluminated) DUT, causing leakage current conducted through defective dielectric features to emit extremely faint visible and infrared light. This emitted light is collected and amplified to obtain an "emitted" light image, which is digitized and stored. Fourth, the digitized background image is subtracted from the digitized emitted image to provide a "difference" image showing defect emission bright spots, with some noise interference remaining. Fifth, the difference image is filtered or processed by an image processing computer to further separate emitted light points from the random noise bright points inherent to the very large signal amplification done in the primary camera. This processing is conventionally done on the basis of light intensity (gray level) threshold discrimination. However, some noise light emissions are more intense, and produce brighter spots, than the defects of interest, and pass the threshold filter even while the threshold is set high enough to block some interesting defect bright spots. This filtering produces a "processed difference" image. Sixth, the sometimes difficult to recognize "processed difference" image is superimposed over the reflected image of the same area so that photon emission spots can be seen and located with respect to the IC. With this information, a process or failure analysis engineer can, afterwards, refer to the composite layout of the IC, determine the probable cause of failure, and correct the IC design.
Emission microscopy has the advantage that it is a non-destructive technique and does not introduce new defects into the DUT, unlike the conventional technique of stripping layers off of the DUT, which can introduce new defects and is extremely time consuming.
However, this prior art emission microscopy implementation is complicated by the fact that for the defects' extremely faint emissions of light to be detected requires observing the emissions through a lens which maximizes brightness. The amount of light transmitted through a lens is proportional to NA.sup.2 /MAG.sup.2, where NA is the numerical aperture, and MAG is the magnification, of the lens. This requires a high numerical aperture and low magnification, but there have not previously been commercially available lenses of sufficient quality meeting this requirement. To obtain sufficient brightness with low magnification, the prior art system used a Nikon objective lens with a NA of 0.025, and 1.times. magnification. Khurana (col. 3 lines 42 through 44) mentions using a lens with NA 0.8 but with a high magnification of 100.times.. When used at a distance practical in a microscope, a lens with this NA limit subtends a field of view only large enough to cover a sub-area, say 1/10, of a typical IC. Thus, the prior art apparatus is only able to examine one sub-area of an IC at a time. Locating defects wherever located in an IC required scanning the entire IC die one sub-area at a time, by successively manually repositioning the stage or optics to line up the camera over each sub-area and repeating the steps of multiple image capture, differencing, and processing for each sub-area, was a cumbersome, error prone, and time consuming process.
Furthermore, it is not uncommon for an IC when stimulated by one test vector to fail due to a defect which only manifests as a consequence of an immediately preceding state during the preceding clock cycle of the IC, when it was stimulated by another test vector. Test vectors cannot be successively set up and applied in real time in the prior art system using manual switches, which generally precludes identifying dynamic failure condition defects.
It is therefore an object of this invention to provide an emission microscopy system which is useable to locate defects in IC's conveniently and quickly.
It is another object to provide a system in which DUT's can be stimulated by application of test vectors in selected sequences and in real time to recreate dynamic failure conditions.
It is another object to provide a process for more precisely locating defects in IC's.